A need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. A need also exists for a replacement for the conventional cathode-ray tube used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Both of these needs can be satisfied by display devices that incorporate a light valve that uses as its light control element a reflective spatial light modulator based on a surface-stabilized ferroelectric liquid crystal (SSFLC) material.
A SSFLC-based reflective spatial light modulator is composed of a layer of a SSFLC material sandwiched between a transparent electrode and a reflective electrode. The reflective electrode is segmented into an array of pixel electrodes to define the picture elements (pixels) of the spatial light modulator. The reflective electrode can be on a silicon substrate that also accommodates the drive circuits that derive the drive signals for the pixel electrodes from the input video signal. The direction of an electric field applied between each pixel electrode and the other electrode determines whether or not the corresponding pixel of the spatial light modulator rotates the direction of polarization of light falling on the pixel. The spatial light modulator is constructed as a quarter-wave plate and rotates the direction of polarization through 90.degree. so that the polarized light reflected by the pixels of the spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel. The resulting optical characteristics of each pixel of the spatial light modulator are binary: the pixel either reflects light (its 1 state) or absorbs light (its 0 state), and therefore appears light or dark, depending on the direction of the electric field.
To produce the grey scale required for conventional display devices, the apparent brightness of each pixel is varied by temporally modulating the light transmitted by each pixel. The light is modulated by defining a basic time period that will be called the illumination period of the spatial light modulator. The pixel electrode is driven by a drive signal that switches the pixel from its 1 state to its 0 state. The duration of the 1 state relative to the duration of the illumination period determines the apparent brightness of the pixel.
Ferroelectric liquid crystal-based spatial light modulators suffer the disadvantage that, after each time the drive signal has been applied to a pixel electrode to cause the pixel to modulate the light reflected by it, the DC balance of the pixel must be restored. This is done by defining a second basic time period called the balance period, equal in duration to the illumination period, and driving the pixel electrode with a complementary drive signal having 1 state and 0 state durations that are complementary to the 1 state and 0 state durations of the drive signal during the illumination period. The illumination period and the balance period collectively constitute a display period.
To prevent the complementary drive signal from causing the display device to display a substantially uniform, grey image, the light illuminating the light valve may be modulated so that the light valve is only illuminated during the illumination period, and is not illuminated during the balance period. One way that the light may be modulated is by modulating the light source itself. Modulating the light source works well with fast-acting light sources such as Light Emitting Diodes (LEDs), but has not worked well with slower-acting light sources, such as arc-lamps. Modulating the light from these slow-acting light sources for use with the ferroelectric liquid crystal based spatial light modulators has been problematic.
FIG. 1A shows part of a conventional display device 5 incorporating a conventional reflective light valve 10 that includes the reflective spatial light modulator 12. Other principal components of the light valve are the polarizer 14, the beam splitter 16 and the analyzer 18. The light valve is illuminated with light from the light source 20, the light from which is concentrated on the polarizer using a reflector 22 and collector optics 24. The light output by the light valve passes to the imaging optics 26 that focus the light to form an image (not shown). The light valve, light source and imaging optics may be incorporated into various types of display device, including miniature, wearable devices, cathode-ray tube replacements, and projection displays.
In general terms, light generated by the light source 20 passes through the polarizer 14. The polarizer polarizes the light output from the light source. The beam splitter 16 reflects a fraction of the polarized light output from the polarizer towards the spatial light modulator 12. The spatial light modulator 12 is divided into a two-dimensional array of picture elements (pixels) that define the spatial resolution of the light valve 10. Light reflected from the spatial light modulator can pass to the beam splitter 16 which transmits a fraction of the reflected light to the analyzer 18.
The direction of an electric field in each pixel of the spatial light modulator 12 determines whether or not the direction of polarization of the light reflected by the pixel is rotated by 9.degree. relative to the direction of polarization of the incident light. The light reflected by each pixel of the spatial light modulator passes through the beam splitter 16 and the analyzer 18 and is output from the light valve 10 through the imaging optics 26 depending on whether or not its direction of polarization was rotated by the spatial light modulator.
More specifically, the polarizer 14 polarizes the light generated by the light source 20 that passes through the collector optics 24 either directly or after reflecting off reflector 22. The polarization is preferably linear polarization. The beam splitter 16 reflects the polarized light output from the polarizer towards spatial light modulator 12, and the polarized light reflected from the spatial light modulator transmits to the analyzer 18 through the beam splitter 16. The direction of maximum transmission of the analyzer is orthogonal to that of the polarizer in this example.
The spatial light modulator 12 is composed of the transparent electrode 28 deposited on the surface of the transparent cover 30, the reflective electrode 32 located on the surface of the semiconductor substrate 34, and the surface-stabilized ferroelectric liquid crystal (SSFLC) layer 36 sandwiched between the transparent electrode and the reflective electrode. The reflective electrode is divided into a two-dimensional array of pixel electrodes that define the pixels of the spatial light modulator and of the light valve. A substantially reduced number of pixel electrodes are shown to simplify the drawing. For example, in a light valve for use in a large-screen computer monitor, the reflective electrode could be divided into a two-dimensional array of 1600.times.1200 pixel electrodes. An exemplary pixel electrode is shown at 38. Each pixel electrode reflects the portion of the incident polarized light that falls on it towards the beam splitter 16.
A drive circuit (not shown), which may be located in the semiconductor substrate 34, applies a drive signal to the pixel electrode 38 of each pixel of the spatial light modulator 12. The drive signal has two different voltage levels, and the transparent electrode 28 is maintained at a fixed potential mid-way between the voltage levels of the drive signal. The potential difference between the pixel electrode and the transparent electrode establishes an electric field across the part of the liquid crystal layer 36 between the pixel and transparent electrodes. The direction of the electric field determines whether the liquid crystal layer rotates the direction of polarization of the light reflected by the pixel electrode, or leaves the direction of polarization unchanged.
Since light passes through the reflective spatial light modulator twice, once before and once after reflection by the reflective pixel electrodes, the reflective spatial light modulator 12 is structured as a quarter-wave plate. The thickness of the layer of ferroelectric liquid crystal material in the liquid crystal layer 36 is chosen to provide an optical phase shift of 90.degree. between light polarized parallel to the director of the liquid crystal material and light polarized perpendicular to the director. The liquid crystal material is preferably a Smectic C* ferroelectric liquid crystal material having an angle of 22.5.degree. between its director and the normal to its smectic layers. Reversing the direction of the electric field applied to such a liquid crystal material switches the director of the material through an angle of about 45.degree.. Consequently, if the director is aligned parallel to the direction of maximum transmission of the analyzer 18 with one polarity of the electric field, reversing the direction of the electric field will rotate the direction of polarization of light reflected by the pixel through 90.degree.. This will align the direction of polarization of the light perpendicular to the direction of maximum transmission of the analyzer, and will change the pixel from its 1 state, in which the pixel appears bright, to its 0 state, in which the pixel appears dark.
In a miniature, wearable display, the imaging optics 26 are composed of an eyepiece that receives the light reflected by the reflective electrode 32 and forms a virtual image at a predetermined distance in front of the user (not shown). In a cathode-ray tube replacement or in a projection display, the imaging optics are composed of projection optics that focus an image of the reflective electrode on a transmissive or reflective screen (not shown). Optical arrangements suitable for use as an eyepiece or projection optics are well known in the art and will not be described here.
Since the direction of maximum transmission of the analyzer 18 is orthogonal to the direction of polarization defined by the polarizer 14, light whose direction of polarization has been rotated through 90.degree. by a pixel of the spatial light modulator 12 will pass through the analyzer and be output from the light valve 10 whereas light whose direction of polarization has not been rotated will not pass through the analyzer. The analyzer only transmits to the imaging optics 26 light whose direction of polarization has been rotated by pixels of the spatial light modulator. The pixels of the spatial light modulator will appear bright or dark depending on the direction of the electric field applied to each pixel. When a pixel appears bright, it will be said to be in its 1 state , and when the pixel appears dark, it will be said to be in its 0 state.
The direction of maximum transmission of the analyzer 18 can alternatively be arranged parallel to that of the polarizer 14, and a non-polarizing beam splitter can be used as the beam splitter 16. In this case, the spatial light modulator 12 operates in the opposite sense to that just described.
To produce the grey scale required by a display device notwithstanding the binary optical characteristics of the pixels of the light valve 10, the apparent brightness of each pixel is varied by temporally modulating the light reflected by the pixel, as described above. The drive circuit (not shown) for each pixel of the spatial light modulator determines the duration of the 1 state of the pixel in response to a portion of the input video signal 40 corresponding to the location of the pixel in the spatial light modulator.
FIGS. 1B-1F illustrate the operation of the exemplary pixel 38 of the conventional light valve 10 shown in FIG. 1A during three consecutive display periods. The remaining pixels operate similarly. In one embodiment of a conventional light valve, each display period corresponded to one frame of the input video signal 40. In another embodiment, each display period corresponded to a fraction of one frame of the input video signal. Each display period is composed of an illumination period (ILLUM) and a balance period (BALANCE) having equal durations, as shown in FIG. 1B.
FIG. 1C shows the drive signal applied to the exemplary pixel electrode 38. The transparent electrode 28 is held at a voltage level of V/2, so that changing the voltage level on the pixel electrode from 0 to V reverses the direction of the electric field applied to the ferroelectric liquid crystal layer 36. The level of the drive signal is V for a first temporal portion 1T of each illumination period. The level of the drive signal is 0 for the second temporal portion 2TP constituting the remainder of the illumination period, and also for the first temporal portion 1T of the subsequent balance period. The first temporal portion of the balance period has a duration equal to the first temporal portion of the illumination period. However, the level of the drive signal is 0 during the first temporal portion of the balance period, whereas the level of the drive signal is V during the first temporal portion of the illumination period. Finally, the level of the drive signal changes to V for the second temporal portion 2TP constituting the remainder of the balance period. Consequently, during the balance period, the level of the drive signal is 0 and V for times equal to the times that it was at V and 0, respectively, during the illumination period. As a result, the electric field applied to the liquid crystal material of the pixel averages to zero over the display period.
In the example shown, the duration of the first temporal portion 1TP of the drive signal is different in each of the three illumination periods. The duration of the first temporal portion, and, hence, of the second temporal portion, of each illumination period depends on the voltage level of the corresponding sample of the input video signal 40.
FIG. 1D shows the effect of the spatial light modulator 12 on the direction of polarization of the light impinging on the analyzer 18. The direction of polarization is indicated by the absolute value of the angle .alpha. between direction of polarization of the light impinging on the analyzer and the direction of maximum transmissivity of the analyzer. The analyzer transmits light having an angle .alpha. close to zero and absorbs light having an angle .alpha. close to 90.degree.. In each display period, the angle a has values corresponding to the pixel being bright and dark for equal times due to the need to restore the DC balance of the pixel.
FIG. 1E shows the condition of a fast-acting light source 20. The light source is ON throughout the illumination period of each display period, and is OFF during the following balance period.
FIG. 1F shows the light output from the exemplary pixel of the light valve 10 controlled by the pixel electrode 38. Light is output from the pixel only during the first temporal portion of the illumination period of each display period. No light is output during the second temporal portion of the illumination period. Moreover, no light is output during the balance period of the display period because the light source 20 is OFF during the balance period.
The light valve 10 can also be adapted to provide a colored light output to the imaging optics 26. One way that this can be done is by either replacing the "white" light source 20 with three colored light sources such as a red, blue and green LEDs, each illuminating the spatial light modulator sequentially. Another way that a colored light output can be provided is by replacing the single spatial light modulator 12 depicted in FIG. 1A with a series of three dichroic plates 42, each having an associated reflective spatial light modulator 12, as shown in FIG. 2. Each of the dichroic plates is configured to reflect light in a band of wavelengths particular to that dichroic plate and to pass the remaining wavelengths of light. Thus, if the light source 20 is a "white" light, emitting visible light across the entire color spectrum, a particular portion of the color spectrum may be reflected by each dichroic plate its associated reflective spatial light modulator simultaneously. This eliminates the need for sequential illumination and improves the perceived brightness of the color pixels passing through the analyzer.
For example, the dichroic plate 42 nearest the beam splitter 16 might reflect red colored light toward its associated spatial light modulator 12 while the center dichroic plate reflects green colored light toward its associated spatial light modulator and the dichroic plate remote from the beam splitter reflects blue colored light towards its spatial light modulator. When the light source 20 is ON, as shown if FIG. 2, the colored light reflected by the dichroic plates passes to each of the three reflective spatial light modulators 12. Each of the three reflective spatial light modulators is capable of reflecting pixels of the colored light back at its associated dichroic plate in a manner consistent with the above description of the operation of the spatial light modulator shown in FIG. 1A.
The majority of the colored light reflected by each of the spatial light modulators 12 will be reflected by associated dichroic plate toward the analyzer 18 since the light reflected by each spatial light modulator will retain the characteristic wavelengths of lights originally reflected by the dichroic plates. When the individual colored light from each of the three reflective spatial light modulators 12 pass through the analyzer, they are combined and a full color image can be formed by the imaging optics 26.
The use of bright "white" light sources with reflective spatial light modulators is desirable, particularly in applications intended to replace the conventional cathode-ray tube used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Most bright "white" light sources tend to be slower-acting, however, and can not be directly modulated effectively. Indirect modulation of bright light sources has been accomplished using mechanical shutters in the motion picture industry. Mechanical shutters include blade, curtain, and plate shutters. Due to their mechanical nature, however, such shutters are often too large, noisy, and power-consuming to be used effectively in may display applications.
A SSFLC-based shutter is also known in the art and is described in U.S. Pat. No. 5,029,987 issued Jul. 9, 1991 and entitled "Ferroelectric liquid crystal shutter." A detailed view of a SSFLC-based shutter 13 is shown in FIG. 3. In general terms, SSFLC-based shutters are composed of a layer of a SSFLC material 44 (shown with hash marks for clarity) sandwiched between an input transparent electrode 52 including an input polarizing layer 58 and an output transparent electrode 48 including an output polarizing layer 46 (polarizing layers shown with hash marks for clarity). The SSFLC-based shutter 13 is constructed as a half-wave plate and depending on the direction of an electric field applied between the input transparent electrode 52 and the output transparent electrode 48, either rotates the direction of polarization of light entering the shutter 13 through 90.degree. or maintains the direction of polarization. Thus, the direction of the electric field applied between the input transparent electrode 52 and the output transparent electrode 48 along with the direction of polarization of the input polarizing layer 58 relative to the direction of polarization of the output polarizing layer 46 determines whether the shutter transmits or blocks light from the light source (not shown). For example, if the direction of polarization of the output polarizing layer 46 is orthogonal to the direction of polarization of the input polarizing layer 58, the shutter 13 will transmit light when the shutter rotates the direction of polarization by 90.degree. (its OPEN state) and will absorb light when the direction of polarization is not rotated (its CLOSED state). Alternatively, if the direction of polarization of the output polarizing layer 46 is parallel to the direction of polarization of the input polarizing layer 58, the shutter 13 will transmit light when the direction of polarization is not rotated (its OPEN state) and will absorb light when the direction of polarization is rotated by 90.degree. (its CLOSED state). The resulting optical characteristics of the shutter are binary: the shutter either transmits light (its OPEN state) or absorbs light (its CLOSED state), depending on the direction of the electric field.
Consequently, what is needed is a reflective ferroelectric liquid crystal-based light valve where the duration of the illumination period is controlled with a shutter.